As an oscillator for obtaining a highly accurate oscillation output, there has been known an oven controlled crystal oscillator (OCXO) including, for example, an oscillator circuit, a crystal resonator, a heater, and a temperature sensor in an oven. The heater is connected to an input terminal for inputting a supply voltage Vcc of the OCXO, for example. The temperature sensor detects a temperature in the oven and outputs a control voltage corresponding to the detected value to the heater. On the basis of this control voltage, a current flowing from the input terminal to the heater is controlled and a heater electric power is controlled so as to keep the temperature in the oven constant. This stabilizes an output frequency from the crystal resonator. That is, the oven is configured to be a constant temperature oven.
The heater, for example, includes a PNP-type transistor or an NPN-type transistor. The control voltage is applied to bases of these transistors. In the case where the PNP-type transistor is used, the input terminal of the supply voltage is connected to an emitter and in the case where the NPN-type transistor is used, the input terminal of the supply voltage is connected to a collector. As described in the embodiment, the heater electric power is a current supplied to these transistors×Vcc.
A supply voltage Vcc may fluctuate. When the supply voltage Vcc thus fluctuates, the heater electric power fluctuates in association with it and the temperature in the constant temperature oven fluctuates. As the result, stability of the oscillation frequency deteriorates. In order to prevent this fluctuation of the heater electric power, it is possible to dispose a regulator between the input terminal and the heater to keep a voltage applied to the heater constant; however, a heat generation caused by voltage drop in this regulator becomes large because the heater requires a current larger than other circuit components to flow. This fails the temperature control when a set temperature in the constant temperature oven is set to a comparatively high temperature.
Patent Document 1 describes a temperature control circuit including the above-described heater and temperature sensor and shows that this temperature control circuit is applied to the OCXO. However, in this Patent Document 1, the temperature sensor is constituted of a thermistor, which is difficult to expect an improvement of frequency accuracy of the OCXO due to a limit of temperature detection accuracy of this thermistor.